Fly Ash Mortars Research Articles

Natural and accelerated carbonation behaviour of high-volume fly ash (HVFA) mortar: Effects on internal moisture, microstructure and carbonated phase proportioning

Binders with large portions of carbon-intensive Portland cement replaced by supplementary cementitious materials (e.g. fly ash) are more susceptible to carbonation mainly due to their lower CO2 buffering capacity. This conclusion is usually drawn from accelerated experiments at elevated CO2 levels involving processes that seriously differ from natural carbonation. The resulting presence of H2O reactant in the pore system and the carbonated microstructure itself may be very different. In this paper, these phenomena were investigated for High-Volume Fly Ash (HVFA) mortar via carbonation tests at ±0.04% CO2 (natural carbonation), 1% CO2 and 10% CO2. Internal humidity sensor monitoring and 1H NMR relaxometry revealed the highest water vapour and liquid water contents after carbonation at 10% CO2. Carbonation at 10% CO2 results in a coarser pore structure than carbonation at 1% CO2, and this probably due to a higher degree of C–S–H carbonation.

Effect of colloidal nanosilica as an admixture on chloride ion diffusion of Portland cement–fly ash mortars

The properties of the ordinary Portland cement (OPC) mortar having a water–cement (w/c) proportion of 0.450 and cement/sand ratio of 1:2 are altered by replacing cement with 25% fly ash and adding 2% colloidal nanosilica separately and in a combined form. The changes were studied experimentally. Cured specimens at different ages were subjected to compressive strength test, split tensile test, and durability tests such as sorptivity, water absorption, and chloride penetration. The impact of the addition of colloidal nanosilica on the permeability of the mortar matrix, estimated by the coefficient of absorptivity, was computed by performing a water absorption test for 60 min. It was observed that in the initial curing regime, fly ash expansion causes a decrease in strength and an increase in water permeability. The addition of 2% colloidal nanosilica proves to be advantageous in the hardened OPC and fly ash mortar as it increases the materials characteristic strength by conferring it a dense microstructure. Nanosilica in the cement matrix with and without fly ash assists the rapid growth of hydrated products by creating additional nucleation sites and improving the durability properties of the material as anodic polarization decreases under impressed voltage. The microstructure was analyzed by SEM and found to have the refinement of a pore structure and densification of binding gels in the colloidal nanosilica -incorporated samples.

Effect of various commercial of Na2SiO3 on compressive strength of Fly ash-based alkaline activated mortar

This study aimed to assesses the effect of various commercial Na2SiO3 on the compressive strength (CS) of alkaline activated fly ash mortar (AAFM). The three mixture of alkaline activated mortar (AAM) C1, C2 and C3 were prepared from the source material of fly ash and alkaline activator solution (AAS). The initial AAS was comprised of NaOH (10M) and various grade of Na2SiO3. The various grades of Na2SiO3 were characterized by their SiO2/Na2O molar ratio of 2.0, 2.2, and 3.3, respectively. The sample from each mixture was characterized based on the CS and microstructure changes using useful tools of XRD and FTIR analysis. The results obtained indicated that the highest CS achieved among the three mixtures were 48.23MPa of mixture C2 prepared with SiO2/Na2O molar ratio of 2.2. This was mainly due to higher binder formation (N-A-S-H gel type) and a higher rate of reaction of the main source material. This result is in line with XRD and FTIR analysis results finding.

Lowering carbon foot print of Portland cement by class F fly ash substitution in mortar mixture in terms of workability and strength properties

In the experimental work, recycling by inclusion of class F fly ash in mortar was investigated in terms of workability and strength of mortar to fabricate an environment friendly construction material. For this aim, class F fly ash were used as cement replacement at 10%, 30%, 50% and 70% ratios in mortar. Water-binder ratios were 0.40, 0.45 and 0.50. A total of 15 different mortar mixtures including control Portland cement and fly ash mortar were produced. Workability of fresh mortar was measured using mini flow testing. Flexural and compressive strength of hardened standard sized samples were measured after completion of their specified curing time at 1d, 3d, 7d 28d, 3m and 6m. Experimental results showed that inclusion of fly ash in mortar improved workability with respect to control Portland cement mortar. Recycling low amount of fly ash in mortar did not show detrimental but beneficial influence on strength properties of mortar. However, employing high amount of fly ash in mortar reduced flexural and compressive strength at early ages, however, the reduction in flexural and compressive strengths were remedied due to pozzolanic reaction of fly ash at longer curing time compared to control Portland cement mortar. It was concluded that current fly ash was a suitable pozzolanic material to be recycled by replacement with Portland cement in mortar to manufacture clean construction material in terms of workability and strength concern.

Influence of Activators on Mechanical Properties of Modified Fly Ash Based Geopolymer Mortars.

The aim of this work was to study the influence of the type of activator on the formulation of modified fly ash based geopolymer mortars. Geopolymer and alkali-activated materials (AAM) were made from fly ashes derived from coal and biomass combustion in thermal power plants. Basic activators (NaOH, CaO, and Na2SiO3) were mixed with fly ashes in order to develop binding properties other than those resulting from the use of Portland cement. The results showed that the mortars with 5 mol/dm3 of NaOH and 100 g of Na2SiO3 (N5-S22) gave a greater compressive strength than other mixes. The compressive strengths of analyzed fly ash mortars with activators N5-S22 and N5-C10 (5 mol/dm3 NaOH and 10% CaO) varied from 14.3 MPa to 5.9 MPa. The better properties of alkali-activated mortars with regular fly ash were influenced by a larger amount of amorphous silica and alumina phases. Scanning electron microscopy and calorimetry analysis provided a better understanding of the observed mechanisms.

Chloride‐induced steel corrosion in alkali‐activated fly ash mortar: Increased propensity for corrosion initiation at defects

AbstractChloride contents at the steel–mortar interface that initiate steel corrosion were determined for carbon steel in alkali‐activated fly ash mortar for three different exposure conditions: exposure to 1 M NaCl solution; leaching in deionized water and then exposure to 1 M NaCl solution; and leaching in deionized water, aging in air at 20°C and natural CO2 concentration, and then exposure to 1 M NaCl solution. For comparison, a Portland cement mortar, exposed to 1 M NaCl solution, was studied. The median values of the corrosion‐initiating chloride contents (average over the full length of the rebar) in the alkali‐activated fly ash mortar varied between 0.35 and 1.05 wt% Cl with respect to binder, consistently lower than what was obtained for the Portland cement mortar, but with no clear trend regarding the exposure conditions. For most of the alkali‐activated fly ash mortar specimens, preferential corrosion at the connection between the working electrode and the external measurement setup was observed, while preferential corrosion did not occur for the Portland cement mortar. Scanning electron microscopy and auxiliary experiments in synthetic solutions indicated that this behavior was caused by inhomogeneities at the steel–mortar interface in the alkali‐activated mortar, likely due to its peculiar rheological properties in the fresh state.

The Threshold Value of Effective Replacement Ratio of Fly Ash Mortar Based on Amount of Calcium Hydroxide

The effective mineral admixture in mortar can consume the calcium hydroxide produced by hydration reaction because of pozzolanic effect. For high volume mineral admixture mortar, when the replacement ratio exceeds a certain ‘threshold’ value, the supply of Ca(OH)2 amount is insufficient, the superfluous and ineffective mineral admixture will no longer react in mortar but as fine aggregate. This study presents an experiment study on the threshold value of effective replacement ratio of fly ash mortar by the comprehensive analysis of Ca(OH)2 amount and consumption curves. Under the conditions of different curing temperature, the effective replacement ratios of fly ash mortar have been determined ultimately. The results showed that Ca(OH)2 amount and consumption in mortar both decreased with replacement ratio. For fly ash mortar, at 20°C curing temperature, when the replacement ratio was less than 40%, the Ca(OH)2 amount decreased obviously with replacement ratio. However, when replacement ratio was more than 40%, Ca(OH)2 amount at 91d changed slightly with replacement ratio, the Ca(OH)2 amount was consumed to a limit value at this time. Under the condition of 30°C curing temperature, when the replacement ratio exceeded 30%, the change of Ca(OH)2 amount with replacement ratio was close to a straight line, the superfluous fly ash no longer consumed Ca(OH)2 basically. Moreover, the consumption of Ca(OH)2 was not much difference to the Ca(OH)2 amount at the same replacement ratio under the condition of 20°C curing temperature. This result has a great significance on effective utilization of mineral admixtures in engineering application.

Influence of fly ash on the pore structure of mortar using a differential scanning calorimetry analysis

In the paper a low-temperature thermoporometry using differential scanning calorimetry (DSC) was employed for analyse of influence of siliceous fly ash (FA) on pore structure of non-air-entrained mortars (pore size, connectivity). A method of interpreting a heat flux differential scanning calorimetry records in pore structure was used for this purpose. The results demonstrated that the: (i) fly ash mortars have virtually no pores inaccessible to water, unlike the mortars with plain Portland cement in which inaccessible pores constitute a significant fraction, growing with the increase in w/b; (ii) with a decrease in w/b the ink-bottle volume decreases. Fraction of this pore type is relatively larger in fly ash mortars; (iii) Siliceous fly ash increased the volume of pores greater than 8 nm, in particular in the group with radii larger than 20 nm at all w/b ratios.

STUDY ON MECHANICAL AND DURABILITY PROPERTIES OF MIXTURES WITH FLY ASH FROM HONGSA POWER PLANT

The use of fly ash in concrete improves several characteristics of conventional cement-based pastes, mortars, and concrete such as reduces heat of hydration, increases strength in long-term and enhances durability. However, types and volume of fly ash affect behavior of resulting pastes, mortars and concrete. In this study, the characteristics of pastes, mortars, and concrete with 20% and 30% binder replacement with a Hongsa fly ash from Laos (FAH3) and two fly ashes from Thailand (FAM and FAB) were studied. Further, mechanical and durability properties of Hongsa fly ash mortars and concrete are investigated through specific gravity, Blaine fineness, normal consistency, setting times, water requirement, strength index, slump and slump retention, compressive strength of concrete with a fixed slump, compressive strength of concrete with a fixed w/b of 0.5, semi-adiabatic temperature, total shrinkage, carbonation depth, H2SO4 acid resistance, rapid chloride penetration (RCP) and chloride distribution. The experimental results show that the Hongsa fly ash contains large amount of non-spherical particles with coarse cavities, leading to high surface area and high Blaine fineness value. Accordingly, Hongsa fly ash was found to have high water requirement. In comparison to the ordinary Portland cement type I (OPC) and Mae Moh fly ash (FAM), the Hongsa fly ash was found to generate lower heat. As a result, the Hongsa fly ash shows its potential in the application of mass concrete. Similarly, the Hongsa fly ash mortar exhibited the lowest carbonation depth when compared to the FAM and FAB mortars. In term of RCPT and chloride distribution test, the Hongsa fly ash concrete shows the lowest Cl⁻ penetrability when compared with Portland cement type I (OPC) concrete, FAM and FAB concretes. Based on the experimental results, the Hongsa fly ash was found to be applicable in concrete works.

Using nano-silica to improve mechanical and fracture properties of fiber-reinforced high-volume fly ash cement mortar

Using nano-silica (NS) in cement-based materials has attracted extensive attention. However, whilst most studies focused on the hydration, fresh, mechanical and durability properties, there is limited information on the effect of NS on the fracture properties of fiber-reinforced high-volume fly ash mortars (HVFAM). In this paper, a series of experiments were carried out to evaluate the effect of NS on the fracture and mechanical properties of polyvinyl alcohol (PVA) fiber-reinforced HVFAM, with the fly ash/binder ratio fixed at 50% by weight. Four NS/binder weight ratios of 0% 0.5%, 1.0% and 1.5%, and four PVA fiber volume dosages of 0%, 0.2%, 0.5% and 1.0% were adopted. Compared to 0.2–1.0 vol% PVA fiber-reinforced HVFAM without NS, the addition of 0.5 wt% NS could further increase the flexural strength, fracture energy, fracture toughness, critical crack tip opening displacement and brittleness index by 3–5%, 8–29%, 17–28%, 18–38% and 34–95%, respectively; the addition of 1.0 wt% NS could increase the flexural strength, fracture energy, fracture toughness, critical crack tip opening displacement and brittleness index by 4–9%, 15–41%, 26–46%, 18–48%, and 62–79%, respectively. The mineralogy and microstructure characterizations showed that the NS condensed the fiber/matrix interface. These findings provide insights into designs and applications of fiber-reinforced high-volume pozzolan cement-based materials with nano-particles.

A model to predict ammonia emission using a modified genetic artificial neural network: Analyzing cement mixed with fly ash from a coal-fired power plant

Selective catalytic reduction (SCR) and selected non-catalytic reduction (SNCR) use ammonia as a catalyst for removing nitrogen oxides in power plants. However, unreacted ammonia in the flue gas denitrification system reacts chemically with SO3 and is adsorbed on the coal fly ash. Unfortunately, fly ash is commonly mixed with concrete because of its technological and economic benefits, without consideration of secondary environmental contamination. We measured the ammonia emissions from different types of fly ash mortar and found they have a strong correlation with the mixing ratio of fly ash, the mortar age, and the size. In order to find out this correlation and thus predict the ammonia concentration under different conditions, we constructed artificial neural network (ANN) models. The comparison results show that the ANN model initialized with the genetic ANN (GANN) algorithm has the smallest root-mean-square error (RMSE) between given and predicted outputs.

Properties of concrete containing fly ash and temperature control measures used during construction

In recent years, fly ash cement has been applied in engineering with the continuous application of concrete admixtures. Although we have been using fly ash concrete and have done a lot of experimental research, but the understanding of its thermophysical properties was not complete, and further research was needed. This paper presented results of a study into important fundamental thermal and physical properties of both fly ash mortar and fly ash concrete. Different replacement percentage of OPC by FA ranging from 0 to 60% by mass were devised. Increasing fly ash content was found to delay the initial setting time and final setting times, decrease both compressive and flexural strengths and reduce hydration heat. It also can effectively reduce the hydration heat evolution of cement-fly ash binder. The adiabatic temperature rise of cement fly ash material in engineering can be estimated by reduction coefficient k. In fly ash concrete tests, thermal properties, including thermal diffusivity, conductivity, specific heat, ultimate tensile strain and static elasticity modulus were also measured and reported. There also appeared the relationship between compressive strength and flexural strength showed similar linear change with the replacement rate of fly ash rising from 30% to 60%. Most importantly, pipe cooling tests of fly ash concrete were carried out and from the perspective of temperature control of mass concrete, dynamic pipe cooling measures were investigated.

Chemical Kinetics Analysis of Strength Development of Mortar Containing Fly Ash Based on Various Curing Temperatures

Due to large size and high internal temperature of hydraulic dams, the strength development process of damming materials is often faster than those under standard curing conditions. In order to further study the strength development features of damming materials under internal temperature conditions, two kinds of fly ash mortars were investigated, of which fly ash content is 40% and 30%. The compressive strength of mortars under three curing temperatures containing 20, 30 and 38°C was tested at 3 days, 7 days, 14 days, 28 days, 56 days and 90 days, respectively. Then chemical kinetics equation was employed to describe the strength development features under different temperature conditions. The results show that the compressive strength of fly ash mortar increases with the increase of curing temperature in the range of 20 to 38 °C. The main reason is that the chemical reaction rate of cementitious materials is accelerated by increasing curing temperature. For example, the hydration rate at 30° is 1.6 to 1.9 times of that at 20°, and the hydration rate at 38° is 3.1 to 3.9 times of that at 20°. In addition, the hydration rate of mortar with higher fly ash content is accelerated more. It can be seen that the apparent activation energy of mortar containing 40% of fly ash is 1.2 times of that with 30% of fly ash, and its chemical reaction barrier is higher. Therefore, the equivalent age of mortar with 40% of fly ash is longer and its compressive strength is improved more greatly under temperature-raising conditions.

Alkali silica reaction of waste glass aggregate in alkali activated fly ash and GGBFS mortars

This paper evaluates the alkali silica reaction (ASR) susceptibility of waste glass aggregate in alkali activated fly ash and ground granulated blast furnace slag (GGBFS) mortars as compared to that in ordinary Portland cement (OPC) mortars. In accelerated mortar bar tests, glass fine aggregate showed much lower expansions in alkali activated fly ash and GGBFS blended mortars than in OPC mortars or alkali activated neat fly ash or GGBFS mortars. Glass aggregate was classified as non-reactive with alkali activated fly ash and GGBFS blends according to 10-day and 21-day expansion limits of the Australian Standard. Microstructural studies revealed that glass aggregate produced typical ASR products in OPC mortars and alkali activated neat GGBFS mortars due to the presence of high calcium. However, alkali activated fly ash and GGBFS blended mortars produced reaction products of low Ca/Si and high Al/Si ratios that reduced the dissolution of reactive silica present in glass aggregate causing less expansions. The observed expansion of the alkali activated neat fly ash mortar is attributed to the analcime phase found in the X-Ray diffraction of this mortar.

Effect of Fineness of Basaltic Volcanic Ash on Pozzolanic Reactivity, ASR Expansion and Drying Shrinkage of Blended Cement Mortars.

This study focuses on evaluating the effect of the fineness of basaltic volcanic ash (VA) on the engineering properties of cement pozzolan mixtures. In this study, VA of two different fineness, i.e., VA fine (VF) and VA ultra-fine (VUF) and commercially available fly ash (FA) was used to partially replace cement. Including a control and a hybrid mix (10% each of VUF and FA), eleven mortar mixes were prepared with various percentages of VA and FA (10%, 20% and 30%) to partially replace cement. First, material characterization was performed by using X-ray florescence (XRF), X-ray powder diffraction (XRD), particle size analysis, and a modified Chappelle test. Then, the compressive strength development, alkali silica reactivity (ASR), and drying shrinkage of all mortar mixes were investigated. Finally, XRD analysis on paste samples of all mixes was performed to assess their pozzolanic reactivity at ages of 7 and 91 days. The results showed increased Chappelle reactivity values with an increase in the fineness of the VA. Mortars containing high percentages of VUF (20% and 30%) showed almost equal compressive strength compared to corresponding FA mortars at all ages, however, the hybrid mix (10% VUF + 10% FA) exhibited higher strength than that of the reference mix (100% cement), particularly, at 91 days. At low percentages (10%), ASR expansion in both VF and VUF mortars was higher compared to the corresponding FA mortar and the opposite behavior was observed at high percentages (20% and 30%). Among all the mixes including the control, mortar with VUF was found to be most effective in controlling drying shrinkages at all ages. The rate of consumption of calcium hydroxide (Ca(OH)2) for pastes containing VUF and FA was almost the same, while VF showed low Ca(OH)2 intensity. These results indicate that an increase in the fineness of VA significantly improvs performance, and therefore, it could be a feasible substitute for commercial admixtures in cement composites.

An experimental and analytical investigation into age-dependent strength of fly ash mortar at elevated temperature

The impact of high temperature on the fresh, age-dependent strength and microstructure properties of low and high content fly ash mortars has been examined in the current investigation. Cement was replaced with fly ash by 0%, 10%, 25%, 40% and 50% on an equal weight basis. The mortar specimens were prepared using blended cement (i.e. cement + fly ash): fine aggregate proportion of 1:3 and water to binder proportion ranging from 0.415 to 0.44 and cured under water for the ages varying from 7 to 90 days. Then, the specimens were dried and then exposed to one heating-cooling cycle of high temperatures starting from 100 °C to 500 °C at an interval of 100 °C for 3 h. The residual strength of blended mortar in compression was measured and compared with plain mortar strength. The morphological characteristics using scanning electron microscopy has also been observed. The blended mortar containing 10% fly ash when exposed up to 500 °C temperatures gives better performance. The microstructure analysis showed that plain mortar at 500 °C is having larger micro-cracks formation and growth than blended mortar containing 10% fly ash. A model is proposed for the prediction of age-dependent compressive strength of blended mortar after heating at high temperature and found good agreement with experiments. The proposed model will be useful in concrete construction and also contribute to sustainable development. Using the proposed model, the age-dependent strength of fly ash mortar at elevated temperature can be predicted, when 28 days strength of plain mortar at ambient temperature is available.

Mechanisms of Alkali-Silica Reaction in Alkali-Activated High-Volume Fly Ash Mortars

Mechanisms of Alkali-Silica Reaction in Alkali-Activated High-Volume Fly Ash Mortars

Mechanical and microstructural properties of alkali-activated slag and slag + fly ash mortars exposed to high temperature

In this study, changes in the mechanical and microstructural properties of alkali-activated blast furnace slag (BFS) and BFS + fly ash (FA) blended mortars exposed to 25, 400, 600 and 800 °C temperatures are investigated. The alkali-activated mortars using the replacement ratios of 0, 25 and 50% FA by weight of BFS together with the activators are produced in addition to the control mortars produced by only cement. Three different optimum Ms (SiO2/Na2O ratio of activators) values of 0.50, 0.75 and 1.00 are determined for the alkali-activated mortars. The FA substitution by BFS increases the high temperature resistance of alkali-activated BFS mortars.

Effect of Calcium Carbonate Whisker and Fly Ash on Mechanical Properties of Cement Mortar under High Temperatures

Calcium carbonate (CaCO3) whisker, as a new type of microfibrous material, has been extensively used in the reinforcement of cementitious materials. However, the combined effect of CaCO3 whisker and fly ash on mechanical properties of cementitious materials under high temperatures was still unknown. In this study, the coupling effect of CaCO3 whisker, and fly ash on mechanical properties of the cement was investigated. Two sets of cement mortars were fabricated, including CaCO3 whisker-based mortar which contained 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.% CaCO3 whisker as cement substitution and CaCO3 whisker-based fly ash mortar which contained 30 wt.% fly ash in addition to 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.% CaCO3 whisker as cement substitution. Mass loss, compressive strength, and flexural strength of these two sets of specimens before and after being subjected to high temperatures of 200°C, 400°C, 600°C, 800°C, and 1000°C were measured. Based on the results of the aforementioned tests, load-deflection test was performed on the specimen which exhibited the superior performance to further study its mechanical behavior after exposure to high temperatures. Moreover, microstructural analysis, such as mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM), was conducted to reveal the damage mechanism of high temperature and to illustrate the combined effect of CaCO3 whisker and fly ash on high-temperature resistance of the cement. Results showed that fly ash could improve the high-temperature performance of CaCO3 whisker-based mortar before 600°C and limit the loss of strength after 600°C.

Influence of treated mud on free shrinkage and cracking tendency of self-compacting concrete equivalent mortars

The present work aims at studying the formulation and characterization of self-compacting concrete equivalent mortars, using calcined mud from the dredged sediments brought from the dam (Western Algeria) and fly ash from the Central Thermal EDF (France). Three SCCEM samples were prepared; a control mortar sample and two mortar samples containing 22% of mineral additions, with a ratio W/B = 0.47. The analysis of the experimental results obtained indicates that mortars comprising calcined mud develop greater compressive strengths than those containing fly ash. Regarding free shrinkage, mortar with calcined mud is characterized by an autogenous shrinkage similar to those of control and fly ash mortars. However, it is more sensitive to total shrinkage and drying as compared to the other mortars. Under the conditions of restrained shrinkage, control and calcined mud mortars are more sensitive to early cracking than mortar based on fly ash.